consisting of 14 turns of 'I4-inch copper tubing surrounding a 4-cm. borosilicate glass tube is employed. The approximate position of the tap is two turns from the grounded end of the coil. -4 standing-wave-ratio meter facilitates initial alignment and provides a direct means of monitoring power. T o align the transmitter, the S. W. R. meter and a dummy load, consisting of a 50-ohm noninductive resistor of appropriate power rating, are attached to t'he transmitter by quarter-wave coaxial cables. Coil L1 and plate output capacitor CBare adjusted to the transmitter frequency and the loading capacitor C3 is adjusted for maximum out'put. After transmitter adjustment, the dummy load is replaced by the output tank, Ls, C4,and the gas pressure in the borosilicate tube is reduced to less than t,wo torr to establish t'he discharge. The exact position of the tap is det'ermined experimentally by varying its position and readjusting C4 until t'he highest ratio of forward to total power is found. The tap is then permanently secured. As the intensity of the discharge is directly related to received power, the brightness of the discharge can be employed in the absence of a S. W.R. meter. h minor adjustment of the loading capacitor C3 may be required to correct for stray capacitance. After completing these adjustments a small variation of Cd will correct for any subsequent changes in gas composition or pressure. If lower output power is
required, it is generally advisable to reduce transmitter output rather than to detune the output tank. RESULTS A N D DISCUSSION
Several devices employing this circuit have been in use for over a year. Gases such as oxygen, hydrogen, carbon dioxide, and methane have been used interchangeably without necessitating readjustment of the electronic system. T o ash organic matter, oxygen is passed through the discharge at a rate of approximately 25 cc. per minute S. T. P. a t a pressure of 1 torr. The specimen to be decomposed is placed in a borosilicate boat 20 cm. beyond the solenoid. Ashing rate is a function of the sample's surface area and mineral content. Three hours is usually sufficient to ash a 1-gram tissue specimen. .is the quantity of organic matter does not measurably alter the electrical characteristics of the system, realignment is not required during the decomposition process. After ashing, the generator is turned off, the gas system returned to one atmosphere, and the solid residue recovered from the combustion boat. A series of cold traps located between the boat and the vacuum exhaust pump may be employed to recover condensable gases.
The system is highly stable. Standing-wave ratios of 1.1 to 1.2 have been routinely achieved. A typical instrument uses an unmodified ,Johnson Viking-I1 amateur radio-transmitter and provides a forward radio-frequency power of 115 watts, with less than one watt reflected. Several transmitters based on a 4CX250 vacuum tube operating as a class-C oscillator have been constructed. These units provide a n output of 250 watts with only 0.25% reflected power. ACKNOWLEDGMENT
Mr. Harvey J. Beaudry assisted in the design and testing of this circuit. LITERATURE CITED
(1) Babat, G. I., J . Inst. Elect. Eng. 94,
(111).2711947).
( 2 i Gleit, C. E.; Holland, W. D., CHEM.34, 1454 (1962).
.~NAL.
(3) Gleit, C. E., Holland, W. D., Wrigley, R. C., Suture 200, 69 (1963). ( 4 ) Jolly, W., "Technique of Inorganic Chemistry," H. B. Jonassen, A. Weissberger, ed., Tol. I, Interscience, Xew York, 1963. ( 5 ) Marsh, H., O'Hair, E., Reed, R . , Wynne-Jones, W. F. K., Nature 198, 1195 (1963). (6) Strong, C. L., Sci. Am. 209, #1, 146 (1963). ( 7 j Thomas, R. S., Fifth Intern. Conf. on Electron Microscopy, Philadelphia, Sept. 1962.
Radioautography and Scanning of Thin layer Chromatograms of Radioactive Water-Soluble Substances R. A. Schwone and R. S. Nokon, Chemistry Department, DePoul University, Chicago, 111.
of thin layer chroof radioactive watersoluble substances for purposes of radioautography and scanning is a problem which has not been solved satisfactorily. Water-base sprays of plastic ( 2 , 3) cause distortion of the spots. Collodion ( I ) or parlodion which use ethanol as a solvent cause diffusion and sometimes migration of the spot. Sprays using hydrocarbons or halogenated hydrocarbons as solvents ( 7 ) dry to form a film which is thin, brittle, and generally difficult to handle. The following method has been found satisfactory for thin layer chromatograms of radioactive sugars and polyhydric alcohols, using silica gel and kieselguhr adsorbents. A stock solution containing 11.2 grams of polystyrene per 100 ml. of benzene with dibutyl phthalate added as a plasticizer is poured over the developed plates which lie horizontally on a bench top. The plates are allowed to gel and then placed in an oven a t 70" C. for half an hour to HL
RLMOVAL
Tmatograms
dry. They are then immersed in water a t 55" C . for 5 minutes and removed. The polystyrene film is stripped off immediately by sliding a razor blade between it and the glass plate. iifter it is pressed flat, the film is ready for scanning or radioautography. Polystyrene crystalline pellets (monomer polymerized with no lubricant, Brand Plastics, Willow Springs, Ill., No. GlC1) are dissolved in benzene to form the stock solution. The concentration noted above allows for rapid drying but also gives sufficient viscosity so that the liquid does not run off the edge of the plate. Styrofoam dissolves more rapidly than the pellets, but forms a foamy solution. The optimum amount of plasticizer depends on the adsorbent. For Silica Gel G , 0.023 ml. of dibutyl phthalate is added to each milliliter of stock solution. For Kieselguhr G, 0.014 ml. is added. If the percentage of plasticizer is increased, the pliability of polystyrene film is improved, but the percentage of beta-
radiation absorbed is also increased. For quantitative work, the percentage of plasticizer should not be varied from one sample to another. A 200-mm. X 50-mm. adsorbent layer (0.008 inch thick) is adequately covered by 10 ml. of stock solution if the adsorbent is Kieselguhr G or by 15 ml. of stock solution if the adsorbent is Silica Gel G. A two-inch razor blade (Atlantic Industrial Corp., Newark) permits removal of a 50-mm. wide strip in one easy motion. I n appearance, the polystyrene film resembles paraffin and the side which was attached to the glass is smooth. Radioassaying the C14 activity from the smooth side apparently obviates differences in activity due to variations in polystyrene film thickness. This allows quantitative determinations of the amounts of solute on thin layer chromatograms. t-nder the conditions noted above, the polystyrene film absorbs about 50% of the beta-activity emanating from the surface of the VOL. 37, NO. 2, FEBRUARY 1965
315
adsorbent. However, this loss is more than offset by the increased counting efficiency which is possible. That is, the polystyrene film can be sectioned and counted in a simple gas flow counter, whereas the glass plate must be assayed by an end-window type counter, unless specialized equipment is available (6). The fact that the polystyrene solution does not disturb the radioactive solute spot was verified by radioautography. After development, the plate with its glass support was covered with a piece of sheet plastic which was secured to the back of the glass with Scotch Tape. A piece of Kodak Type K K x-ray film was sandwiched between the plasticcovered adsorbent and another piece of support glass. Strong rubber bands were used to hold the sandwich firmly together. The x-ray film was exposed to the thin layer chromatogram for 12 hours and developed by the reconimended methods. The sheet plastic was then removed and the polystyrene mixture was poured over the chromatogram. The smooth side of the resulting polystyrene film was placed in contact with x-ray film in a press (4). The
exposure time was increased to compensate for the polystyrene absorption. The two autoradiograms were compared visually. X o distortion or migration of the spot was observed for any of the radioactive solutes investigated (glucose, mannose, mannitol, and sorbitol). The advantage of using the polystyrene film for radioautography is the ease in handling of the chromatogram. The disadvantages are the loss in sensitivity due t o absorption by the polystyrene and the fact that the solute material cannot be conveniently recovered. If recovery of the material is necessary, then exposure of the x-ray film to the chromatogram on the glass plate is the best method of radioautography. Some workers (5) place the film in direct contact with the adsorbent. However, the plate can be handled without disturbing the adsorbent layer if it is covered with sheet plastic. Sheet plastics tested for this use and the percentage of C14 beta-radiation absorbed by each of them are: 0.00025 Mylar (22%), 0.0005 Mylar (39Y0), Saran Wrap (&Yo) and Handi-Wrap (24%). Handi-Wrap was chosen for
the work above because of its low percentage of absorption and because i t produced a cover free of wrinkles. These techniques provide simple, lowcost methods of radioautography and radioassay which have general application in thin layer chromatography but are particularly useful for water soluble substances. LITERATURE CITED
(1) Barrollier, J., Naturwissenschaften 48,
404 (1961).
( 2 ) Csallany, A. S., Draper, H. H., Anal. Biochem. 4. 418 11962). I , (3) Lichtenbkrger, W., Z. Anal. Chem. 185(2), 111 (1961).
(4) Lotz, W. E., Gallimore, J. C., Boyd, G. A., iVucleonics 10, 28 (March 1952).
(, 5,) Mangold. H. K.. Kammereck. R.. "Proc&dings-l961' International 'Syrn:
posium on Microchemical Techniques," Vol. 2, p. 697, John Wiley, New York, 1962. (6) Randerath, K., "Thin-Layer Chromatography," p. 65, Academic Press, h'ew York, 1963. (7) Squibb, R. L., N a t w e 198, 317 (1963). WORKsupported in part by a grant from the National Science Foundation.
Preparation of Nonmetallurgical Specimens for Electron Probe Microanalysis
T. E.
Reichard and W . S. Coakley, Monsanto Company, St. Louis, Mo.
1"
ELECTRON probe microanalysis of nonmetallic materials the specimens often require specialized handling because of small size, irregular shape, fragility, porosity, low physical stability and/or low electrical conductivity. The following techniques were developed and found useful for electron probe studies of diverse materials such as catalyst pellets, contaminant particles, pigment dispersions, "hard" biological specimens, detergent granules, and semiconductor materials. Analysis of Existing Surfaces. A cylindrical aluminum pill with a suitable depression is used for mounting small beads, granules, "grit," bits of metal, etc., which are t o be retrieved after analysis. T h e depression is almost filled with a soluble gluee.g., Duco cement or a solution of Formvar or Parlodion-premixed with graphite to a paste consistency. The particles are placed on the paste and pressed flush with the outer rim of the pill. The graphite-glue hardens to a stable mounting with high electrical conductivity, and the specimen(s) can be dissolved free after analysis. As a quicker alternative for analysis of microscopic amounts of grit or specks which need not be retrieved, the particles are pressed directly into the flat surface
316
ANALYTICAL CHEMISTRY
Plllrtlc Box
Spec1 men G r a n u 1es
" R e t a i n i n g Wall" (lapel
/'
..,
DOUble-Adh's/Ve Tape
Figure 1. Vacuum-imbedding nique for void-free mountings
tech-
of a solid aluminum pill, using a briquetting press. When ends or edges of elongated or lamellar particles are to be analyzed, a piece of double-coated transparent tape (Scotch Brand Yo. 665, adhesive on both sides) is placed on a microscope slide, and the specimen surface of interest pressed into the adhesive. A short cylindrical ring cut from */r-inch diameter aluminum, copper, or brass tubing is centered around the sample, and also pressed into the adhesive. The open space is filled with indium metal chips which, because of their softness, can be hand pressed and molded around the specimen without displacing or damaging it. The tape is then peeled off and
the mounting is solvent cleaned for analysis. Analysis of Cross Sections. Electrically conductive epoxy resins (loaded with finely divided silver, graphite, or aluminum) are very useful as imbedding media. They harden without heat or pressure and do not damage fragile specimens. Heavily silver-loaded epoxies are generally preferred for their high thermal and electrical conductivity ; however, they produce a relatively high level of continuous x-ray emission. When minimal background emission from the mounting is an important consideration (as for improved contrast in x-ray scanning images) graphite or aluminumloaded epoxies are used. Conductiveepoxy imbedding is especially helpful for analyses of particles with coarse, porous structures wherein a conventional vapordeposited aluminum or carbon coating may fail to form a continuous electrically conductive surface layer. When several specimens are to be studied in cross section, several 'i4-inch rings are placed on a small metal plate and half-filled with epoxy mixture. Specimens are then placed in desired orientations and epoxy added to fill the rings, stirring somewhat to wet the
